There were still some "unresolved issues", he said in reference to media reports that German spies had been operated by CIA agents based at the US embassy in Vienna.

He has agreed with Kerry and US ambassador Alexa Wesner that "the responsible persons will be available for experts from the ministry of the interior and the ministry of defence to clarify pending issues."

"Maybe the matter will be resolved, maybe it won't, and if so we will take appropriate action," Kurz added.

There have been allegations, raised by the German news magazine Der Spiegel, that a US spy who worked for German foreign intelligence (BND) has met at least two CIA agents working at the US embassy in Vienna several times since 2012.